This invention relates to a method of chemically vapor depositing cobalt silicide films. More particularly, the method relates to the chemical vapor deposition of cobalt silicide films in a manner wherein the ratio of cobalt to silicon can be precisely controlled.
Cobalt silicide is a metal with potentially large applications, for example, as a Schottky barrier gate metal, and specifically, on gallium arsenide devices (GaAs), and as a gate and/or contact metal for silicon integrated circuits. Further, cobalt silicide has a close lattice match, e.g., about 1.2 percent difference, to silicon which makes epitaxial growth of cobalt silicide on silicon possible. One potential application or such cobalt silicide epitaxy is a new type of transistor, generally known in the art as a semiconductor-metal-semiconductor transistor as well as epitaxial films for three-dimensional integrated circuits.
Up to now there has been no practical method for performing chemical vapor deposition of cobalt silicide as films, for example, on semiconductor devices. In such applications it is preferable that chemical vapor deposition be conducted because, for example, in the case of depositing thin films on GaAs, the traditional method of sputtering can result in extensive ion and UV photon damage to the GaAs surface as well as the introduction of trace impurities. Even on silicon devices where ion damage is not as extensive a problem, good CVD processing is preferred over sputtering because of the superior coverage achieved with CVD.
In the past, Schottky barriers have been used as gates in metal semiconductor field-effect transistors (MESFETs). In the case of devices that do not undergo high temperature processing, Al or Au can be used as the gate metal. Furthermore, for very small, high speed discrete or integrated GaAs devices, it is desirable to make what is called a self-aligned gate. The process to achieve this involves depositing a thin metal film and then patterning the film to form the gate metal structure. The gate metal becomes the master in a subsequent ion implant to form the source and drain regions. A problem with such a process however is that ion implantation causes damage to the GaAs which must be later removed by annealing at 850.degree. C. for about 30 minutes. During the annealing step, the gate metal must not react or diffuse into the GaAs. Thus, Al, Au, or other non-refractory simple metals cannot be used because they are unstable on GaAs above about 500.degree. C.
It is thus preferred that metal silicides be used because they are more stable than simple metals on GaAs. For example, tungsten silicide, molybdenum silicide and titanium silicide have been tried as metals for making self-aligned gates. It has been found that tungsten rich (W.sub.5 Si.sub.3) tungsten silicide was stable on GaAs at 850.degree. C. W.sub.5 Si.sub.3 was thus one of the first metals found to be useful for ion-implanted self-aligned gates. On the other hand, W.sub.5 Si.sub.3 is not a good solution however because it has, after annealing at 850.degree. C., a relatively high resistivity of about 200 micro ohms-cm which is about two orders of magnitude higher than gold. As a result, the high resistivity can result in reduced device speeds.
It has been proposed that cobalt silicide or nickel silicide be tried for self-aligned gates on GaAs. From thermodynamic considerations, it has been calculated that cobalt silicide or nickel silicide should be thermally stable on GaAs. Further, initial experiments using cobalt silicide appear to confirm this prediction. Still further, cobalt silicide, e.g., CoSi.sub.2 in particular, has a lower resistivity than W.sub.5 Si.sub.3.
In the past the problem with depositing cobalt silicide has been that up to now no effective method of depositing cobalt silicide by chemical vapor deposition, in a manner wherein the ratio of cobalt to silicon can be precisely controlled, has been achieved. For example, in "Chemical Vapour Deposition of Transition-Metal Silicides By Pyrolsis of Silyl Transition-Metal Carbonyl Compounds", J. C. S. Dalton, page 2058, March 31, 1977, Aylett and Colquhoun purpose chemically vapor depositing cobalt silicide from a single precursor comprised of [Co(CO).sub.4 (SiH.sub.3)] to result in a cobalt silicide film. The problem with using this single precursor is that the ratio of cobalt to silicon cannot be precisely controlled and thus, the resultant film has a composition that makes it effectively useless for the specific applications contemplated herein.
Accordingly, the invention avoids the aforementioned problems and provides an effective method of chemically vapor depositing cobalt silicide films in a controllable manner wherein the ratio of cobalt to silicon can be easily controlled.